The invention relates to an apparatus for digital subtraction angiography in the energy subtraction mode with a monochromator for producing two monochromatic X-rays of different energy, with a detector arranged in the optical path of the X-rays behind the monochromator and with a control circuit.
Such an apparatus is already known from Nuclear Instruments and Methods in Physics Research 222, 1984, pp 308-318, North Holland Publishing Company, Amsterdam. It is used for investigating the moving organs of a patient on which an abnormality is assumed. An example is the investigation of the heart to establish whether there is a likelihood of an acute closure of a coronary due to a blood clot. For this purpose an iodine contrasting agent is injected into a vein of the patient's arm and the patient is successively irradiated in linear manner with two linear collimated X-rays, whereof one has an energy E.sub.1 just below the iodine absorption edge of 33 keV and the other an energy E.sub.2 just above the iodine absorption edge. The two X-rays are alternately let through to the same point of the detector through time-controllable beam closures. As the absorption of the X-rays by iodine above the absorption edge is six times as great as below it, but the absorption by the tissue and bones remains the same, there is a clear iodine contrast on subtracting from the first image obtained with energy E.sub.2 from the second image obtained with the energy E.sub.1. However, for this purpose a very intense X-radiation is required, which can e.g. be obtained as synchroton radiation on high energy accelerators or storage rings, such as at DESY or SLAC in Stanford.
A disadvantage of the known apparatus is that it is necessary to have rapidly moving parts for the beam closure in order to release the X-ray and as a result of the necessary down times, the patient cannot be continuously imaged at beam reversal.